We spent three weeks in NZ (not quite long enough) and although we loved the tropical feel of the North Island, most of the memorable highlights were on South Island ... whale watching in Kaikoura (we stayed in the Old Convent B&B which I recommend, if still there, and you fancy a night out of the camper van), seeing the blue penguin colony at Omauru, and Akaroa which is the most charming place we've ever been. We also really regretted not seeing the glow-worm caves at Waitomo, but didn't have time and if we could go back (unlikely) would definitely do that. Generally, just driving and enjoying the gorgeously quiet atmosphere and having the scenery almost always to ourselves except when staying with family/friends was a blast.
Enjoy your holiday, it's a truly fantastic place to visit and *very* easy to fall in love with.